1. Field of Invention
The present invention relates to catalysts, articles, and methods for treating combustion exhaust gas, specifically selective catalytic reduction (SCR) of NOx in lean-burn combustion exhaust gas.
2. Description of Related Art
The largest portions of most combustion exhaust gases contain relatively benign nitrogen (N2), water vapor (H2O), and carbon dioxide (CO2); but the exhaust gas also contains in relatively small part noxious and/or toxic substances, such as carbon monoxide (CO) from incomplete combustion, hydrocarbons (HC) from un-burnt fuel, nitrogen oxides (NOx) from excessive combustion temperatures, and particulate matter (mostly soot). To mitigate the environmental impact of exhaust gas released into the atmosphere, it is desirable to eliminate or reduce the amount of these undesirable components, preferably by a process that, in turn, does not generate other noxious or toxic substances.
One of the most burdensome components to remove from a lean burn exhaust gas is NOx, which includes nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O). The reduction of NOx to N2 in a lean burn exhaust gas, such as that created by diesel engines, is particularly problematic because the exhaust gas contains enough oxygen to favor oxidative reactions instead of reduction. However, NOx can be reduced in a diesel exhaust gas by a process commonly known as Selective Catalytic Reduction (SCR). An SCR process involves the conversion of NOx, in the presence of a catalyst and with the aid of a reductant, into elemental nitrogen (N2) and water. In an SCR process, a gaseous reductant such as ammonia is added to an exhaust gas stream prior to contacting the exhaust gas with the SCR catalyst. The reductant is absorbed onto the catalyst and the NOx reduction reaction takes place as the gases pass through or over the catalyzed substrate. The chemical equation for stoichiometric SCR reactions using ammonia is:2NO+4NH3+2O2→3N2+6H2O2NO2+4NH3+O2→3N2+6H2ONO+NO2+2NH3→2N2+3H2O
Known SCR catalysts include zeolites and other molecular sieves. Molecular sieves are microporous crystalline solids with well-defined structures and generally contain silicon, aluminum and oxygen in their framework and can also contain cations within their pores. A defining feature of a molecular sieve is its crystalline or pseudo-crystalline structure which is formed by molecular tetrahedral cells interconnected in a regular and/or repeating manner to form a framework. Examples of molecular sieves frameworks that are known SCR catalysts include Framework Type Codes CHA (chabazite), BEA (beta), and MOR (mordenite). Catalytic performance of these molecular sieves may be improved in certain environments by a cationic exchange process wherein a portion of ionic species existing on the surface or within the framework is replaced by transition metal cations, such Cu2+. In general, a higher metal loading on such molecular sieves can result in decreased durability of the material, particularly when the material is exposed to high temperatures. Accordingly, there remains a need for more durable, high performance SCR catalysts.